Avoid `reinterpret_cast<uint16_t*>` when computing checksum
Avoid `reinterpret_cast<uint16_t*>` when computing checksum

Correct a potential alignment problem by replacing access to a `uint16_t` through a `reinterpret_cast` with access through a `memcpy`. This generates identical code on x86_64, ARMv7, and ARMv8, but it’s safe and portable to platforms that don’t support unaligned access.

PiperOrigin-RevId: 451467992

google/quiche,google/quiche,google/quiche,google/quiche